1. Field of the Invention
The present invention relates to a chain formed by a plurality of inner links and outer links coupled together alternately and pivotally in the longitudinal direction of the chain and placed over a sprocket.
The chain is used as a transmission chain, for example.
2. Description of the Related Art
Conventional chains that are placed over a sprocket are formed by a plurality of inner links and outer links. The inner link, for example, has a pair of inner link plates and a pair of bushings press-fitted in the link plates. The outer link has a pair of outer link plates and a pair of connecting pins press-fitted in the link plates. The connecting pins of each outer link are loosely fitted in the bushings of each inner link so that the inner links and the outer links are alternately and pivotally coupled together in the longitudinal direction of the chain.
One known chain uses inner links that are formed as a single piece component to include a pair of inner link plates and a pair of bushings by a single piece forming process (see, for example, second to fourth columns and drawings of Japanese Examined Utility Model Application Publication No. H2-10848).
Referring to FIG. 7 and FIG. 8, the conventional chain 500 has inner links 510 and outer links 520. The inner link 510 is formed by a pair of inner link plates 511, and a pair of bushings 512 rotatably supporting rollers 514 and press-fitted into a pair of bushing holes 513 in each inner link plate 511. The outer link 520 is formed by a pair of outer link plates 521 and a pair of connecting pins 522 loosely fitted in the bushings 512 and press-fitted into a pair of pin holes 523 in each outer link plate 521.
As the chain 500 moves, a tensile force is applied in each inner link 510 to the inner link plates 511 by the pair of bushings 512 pressing arcuate portions 511a of the inner link plates 511 in non-facing regions in directions of the tensile force. In each outer link 520, similarly, the tensile force is applied to the outer link plates 521, by the pair of connecting pins 522 pressing arcuate portions 521a of the outer link plates 521 in facing regions in directions of the tensile force.
In such a configuration where the bushings 512 apply the tensile force to the inner link plates 511 in the non-facing regions and the connecting pins 522 apply the tensile force to the outer link plates 521 in the facing regions, when the respective link plates 511 and 521 are subjected to the tension, the tensile stress is concentrated (hereinafter, “concentrated tensile stress”) at boundary portions A1 and A2 including boundaries B between facing regions and non-facing regions in hole peripheral portions 515 around the bushing holes 513 of the inner link plates 511 and in hole peripheral portions 525 around the pin holes 523 of the outer link plates 521.
The facing region is an area in the inner link 510 formed along the longitudinal direction of the chain by facing portions of the pair of bushings 512 facing each other in the longitudinal direction of the chain. The non-facing region is an area located between the facing regions of inner links 510 adjacent each other in the longitudinal direction of the chain. The boundary portions A1 and A2 are portions suffering the concentrated tensile stress around the boundaries B between the facing regions and non-facing regions in the respective hole peripheral portions 515 and 525.
The tensile stress concentrated at the boundary portions A1 and A2 is partly caused by bending deformation of the arcuate portions 511a and 521a as the bushings 512 and the connecting pins 522 press the inner link plates 511 and the outer link plates 521, respectively.
The smaller the cross-sectional area of the respective link plates 511 and 521 at the boundaries, the larger the concentrated tensile stress. In the case with the illustrated chain 500, the cross-sectional area of the inner link plate 511 is minimum at the boundaries B in the facing regions, and the cross-sectional area of the outer link plate 521 is minimum in the non-facing regions.
During the long term use of the chain 500, cracks may be formed in the boundary portions A1 and A2 as they are repeatedly subjected to the concentrated tensile stress caused by fluctuations in chain tension, and there is a possibility of fatigue failure due to rupture of the inner link plates 511 or outer link plates 521. An attempt to improve the fatigue strength to prevent such fatigue failure by increasing the rigidity of the respective link plates 511 and 521 would lead to another problem of unwanted increases in size and weight of the link plates 511 and 521.
The concentration of tensile stress occurs also in a chain that uses inner links formed in one piece including a pair of inner link plates and a pair of bushings as shown in Japanese Examined Utility Model Application Publication No. H2-10848, at joint parts between the inner link plates and the bushings, which are portions corresponding to the boundary portions A1 of the chain 500.